Conventionally, a semiconductor laser is used as a light source for optical communication. A part of a laser light emitted from the light source is reflected by one or more optical components such as an optical connector and the like disposed on an optical path. When a reflected return light (or reflection return light, or external optical feedback), that is, a light reflected by the optical components and returning backward, is incident on the semiconductor laser as the light source, return light induced noises or external optical feedback induced noises are produced within the semiconductor laser. That is, optical output level of the semiconductor laser fluctuates. When the optical output fluctuates, there arises a possibility of transmission code error.
As a method of preventing the reflected return light from entering the semiconductor laser, it is considered possible to provide an optical isolator on the side of the emission end or the outlet end of the semiconductor laser. However, when the optical isolator is used, the optical isolator is itself expensive, and manufacturing process of the light source also becomes complicated, so that manufacturing cost of the light source becomes high.
Therefore, a DFB laser is proposed in which generation of the return light induced noises can be suppressed without using the optical isolator. One example of such DFB laser is disclosed in a document 1, i.e., Japanese patent laid-open publication No. 4-17384 (Japanese patent application No. 2-120026). According to a technique disclosed in this document 1, a DFB laser, in which optical feedback or light feedback is performed by using a diffraction grating, is divided into two regions along the length of a resonator thereof. Also, one of the regions on the side of the emission end is used as a non-excitation region, and the other region is used as an excitation region, that is, a current injection region. Therefore, an electrode for injecting current are provided only on the upper surface of the excitation region. By using such structure, it is possible to utilize a diffraction grating of the non-excitation region as a distributed reflector. As a result, it is possible to prevent the reflected return light from coming into an active layer of the DFB laser.
However, in the technique disclosed in the above-mentioned document 1, reflectance, of the distributed reflector in the non-excitation region, for the output emission light of the DFB laser is the same as reflectance for the reflected return light. As a result, when the reflectance of the distributed reflector is made high, optical loss in the non-excitation region also becomes large, and an oscillation threshold of the DFB laser becomes high. Therefore, it becomes difficult to sufficiently suppress incidence of the reflected return light into an active layer.